In manufacture of pneumatic tires reinforced with cords of steel wire, particularly tires with radially directed steel cord carcasses and circumferential steel cord belts, it is important for proper service that the steel cords be firmly bonded to the rubber and remain bonded until the tires have served their entire useful life, as made or after retreading.
It has been known for many years that rubber can be inseparably bonded to steel by many different procedures. One of them is brass plating the steel and vulcanizing a rubber composition containing sulfur in contact with the brass surface. Accordingly, it is common practice to brass plate the steel wires which are to be made into cords, and then to subject the wires to the final drawing operation for simultaneously reducing the wire diameter and the thickness of the plate, before cabling the wire into the desired cord. Cord ply stock is then made by calender coating with rubber containing sulfur, and such cord ply stock is built into a tire and vulcanized.
It is also known that there are a number of factors critical to success of the brass plate bonding process, including copper content of the brass plate, thickness of the plate, sulfur content of the rubber composition, proper choice of vulcanization accelerator and activator, and time and temperature of vulcanization. In general, a variation in any one of these factors has required an offsetting change in one or more of the others in order to maintain continuity and strength of the adhesive bond.
Moreover, it is known that strength and security of the adhesive bond to brass plate can be enhanced by a presence of a fine silica pigment and of the ingredients for a resorcinol resin.
There are also numerous other known procedures for bonding rubber to steel, ranging from placing them in contact while the metal is red hot, to use of sulfuric acid or other highly corrosive materials, or placing a catalytically active compound such as a cobalt salt or cobalt soap at the interface. None of them consistently produces adhesion of the high tenacity and permanence of properly controlled brass plate adhesion.
In spite of all of the intensive investigation and careful controls of brass plate adhesion which have been carried out by many people over years of time, there are still occasions when the adhesive bond unaccountably fails or is at least weaker than it should be.